The present invention relates generally to a method of MR imaging and, more particularly, to a system and method of PROPELLER magnetic resonance imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Fast Spin Echo (FSE) imaging is an imaging technique commonly used as an efficient method of collecting MRI data with minimal artifact. Generally, FSE requires that the refocusing B1 pulses be applied between each echo such that their phase is substantially identical to that of the initial spin phase after excitation, commonly referred to as the “CPMG” condition. If this condition is not met, the resulting MR signal is general highly sensitive to the strength of B1, and therefore will generally decay rapidly in successive echoes.
As a result, FSE imaging with diffusion weighted imaging (DWI) may be difficult, in general, since even minute patient motion during application of diffusion weighting gradients leaves the spins with a spatially varying and unknown starting phase prior to the spin-echo train. A number of imaging techniques have been developed that alters the phase of the refocusing pulses to attempt to delay the inevitable signal decay. However, these known techniques have been shown to prolong the signal magnitude, but, in general, cause a spatially varying phase which alternates between successive echoes, i.e., the signal in odd echoes encode an additive phase α(x,y), and even echoes encode the opposite phase −α(x,y). This makes combining the two sets of echoes difficult.
FSE imaging is an imaging technique that has been implemented with a number of pulse sequence designs. For example, one FSE technique, which is commonly referred to as Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging, encodes an MR signal by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution.
Another FSE technique, for example, which is commonly referred to as TURBOPROP, has been developed that acquires MR data from multi gradient echoes in each echo spacing. However, image artifacts may be visible in the reconstructed images that may be caused by off-resonance magnetization, susceptibility, gradient delay, or eddy currents.
It would therefore be desirable to have a system and method capable of acquiring multi-blade data in each echo spacing while reducing image artifacts.